Certain limitations on the flexibility of small animal inhalation exposure systems are overcome by the machine control and monitoring of the concentration of the gas or vapor of interest. Computer assistance of chamber operation allows the user to simulate time-varying concentration profiles accurately and repeatedly. We exposed rats to 7 different profiles in which the maximum concentration of carbon tetrachloride (CC14) was 1500 ppm and the concentration times-time (C x T) was 4500 ppm-hr. The purpose was to determine the effects of systematically varying the shape of the concentration profile on the expression of hepatotoxicity of a chemical about which much was already known. All of the exposures were conducted within a span of 6 hours. Examination of the severity of vacuolation and pattern of necrosis could be used to distinguish some of the exposure profiles from others. For example, vacuolation was less severe when 2 equal pulses were presented with an interval of 60 minutes, rather than 180-240 minutes. The indexes of necrosis varied in a more complex way and the differences among the profiles that accounted for the differences in the patterns of the histopathological changes were not immediately apparent. We concluded that the characteristic of a time-related variation in concentration is one of the determinants of the inhalation hepatotoxicity of CC14 and that the simple, time-weighted average concentration may not always fairly represent the best model for the study of problems in inhalation toxicology.